Ch-6 · Title: Ch-6.pmd Author: gbp Created Date: 8/20/2009 5:57:29 PM

1Class-VIII Physics Question Bank

I. Tick (✔✔✔) the most appropriate answer.1. When an ebonite rod is rubbed with fur, the charge acquired by the

fur is:(a) negative (b) positive(c) both positive and negative (d) none of these

2. When a negatively charged body is brought near a suspendedpositively charged ball, the ball gets:(a) attracted (b) repelled(c) stay at same place (d) none of these

3. Aluminium metal is a:(a) good conductor of electricity (b) magnetic substance(c) bad conductor of electricity (d) bad conductor of heat

4. A positively charged body has:(a) excess of electrons(b) deficiency of electrons(c) no change in number of electrons(d) none of these

5. When an uncharged conductor gets electrically charged whenbrought near a charged body the process is called(a) conduction (b) convection (c) induction (d) none of these

6. When a glass rod is rubbed with silk, both glass rod and silk :(a) acquire equal and similar charges(b) acquire equal and opposite charges(c) acquire unequal and similar charges(d) acquire unequal and opposite charges

Ans. 1. (b) 2. (a) 3. (a) 4. (b) 5. (c) 6. (b)

II. Fill in the blanks.1. Cat’s skin acquires __________ charge, when rubbed with ebonite

rod, because its electrons are transferred to __________ rod.

STATIC ELECTRICITY6

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2. Similar charges ____________ each other, whereas oppositecharges _________ each other.

3. It is ______________ electrons in the outermost ______________of an atom, which are responsible for electrification.

4. When a negatively charged body is placed in contact with the_____________ charged disc of G.L.E. its ___________ diverge.

5. In an atom number of ___________ is equal to number of electrons.6. An uncharged insulator can be charged by _______________ with

another body.7. An uncharged conductor can be charged by ______________.8. ______________ is the surest test of electrification.9. A G.L.E. can be charged ________________ by touching a charged

glass rod with its cap.10. A negatively charged body has ______________ of electrons.11. The process of charging objects like glass and amber is known as

___________.12. Ebonite rod acquires ______________ charge when rubbed with

wool.13. A body which has lost electrons becomes ______________.14. ______________ is an instrument used for testing whether a body

is charged or not.15. _____________ is a device that protects buildings from a lightning

strike.16. ______________ is used in the chimneys of factories to prevent

air pollution.17. A glass rod is rubbed with silk, the charge on the glass rod is

____________ and the charge on the silk is _____________ .18. An ebonite rod is rubbed with fur. The charge on the ebonite rod is

___________ and the charge on the fur is ______________.Ans. 1. Positive, ebonite 2. repel, attract 3. free, shell

4. negatively, leaves 5. Protons 6. rubbing7. induction 8. repulsion 9. positively

10. excess 11. electrification 12. negative





13. positively charged 14. electroscope15. lightning conductor 16. electrostatic precipitator17. positive, negative 18. negative, positive

III. Statements given below are incorrect. Write correct statements.1. Glass wool is good conductor of electricity.

Ans. Glass wool is bad conductor of electricity.2. positive electrification is due to the excess of electrons.

Ans. Positive electrification is due to deficiency of electrons.3. When silk is rubbed with glass, the silk gets positively charged.

Ans. When silk is rubbed with glass, the silk gets negatively charged.4. Similar charges attract each other.

Ans. Similar charges repel each other.5. When ebonite rod is rubbed with wool, they get oppositely charged

due to induction.Ans. When ebonite rod is rubbed with silk, they get oppositely charged

due to conduction.6. Lightning conductor always develops the similar charges as is the

charge in the clouds.Ans. Lightning conductor always develops the opposite charges as is the

charge in the clouds.7. A gold leaf electroscope can be charged by both conduction only.

Ans. A gold leaf electroscope can be charged by conduction and induction.8. The electrons flow from a positively charged body to negatively

charged body when connected with a copper wire.Ans. The electrons flow from a negatively charged body to the positively

charged body when connected with a copper wire.9. The fur gets negatively charged because of deficiency of electrons.

Ans. The fur gets negatively charged because of excess of electrons.10. Bad conductors have excess of free electrons.

Ans. Good conductors have excess of free electrons.11. Similar charges attract each other and opposite charges repel each other.

Ans. Similar charges repel each other while opposite charges attract eachother.





12. An uncharged electroscope can tell us the nature of charge on abody.

Ans. A charged electroscope can tell us the nature of charge on a body.13. Lightning is caused due to discharge of similar charged clouds.

Ans. Lightning is caused due to discharge of oppositely charged clouds.14. Lightning do not strikes at high rise buildings.

Ans. Lightning strikes at high rise buildings.15. An insulator allows readily an electric current to pass through it.

Ans. A conductor allows readily an electric current to pass through it.IV. (a) Match the columns.

Column A Column B1. Electric current (a) unit of current.2. Conductor (b) device that prevents

a circuit from overheating3. Insulator (c) flow of electrons along

a conductor.4. Ampere (d) substance which allows

electricity to pass through it.5. Fuse (e) non-conductor.

Ans. 1. (c), 2. (d), 3. (e), 4. (a), 5. (b)(b) Column A Column B

1. Two similar charges (a) acquires a positive charge2. Two dissimilar charges (b) insulator3. When a glass rod is rubbed

with silk, it (c) conductor4. When an ebonite rod is rubbed

with fur, it (d) acquires a negative charge5. Silver (e) repal each other6. Sulphur (f) attract each other

Ans. 1. (e), 2. (f), 3. (a), 4. (d), 5. (c), 6. (b)V. Find the odd one out. Give a reason for your choice.

1. Mica, copper, wood, shellac, sealing wax.Ans. Copper. Copper is a conductor while mica, wood, shellac and sealing

wax are insulators.





2. Mercury, sulphur, graphite, copper.Ans. Sulphur. Sulphur is an insulator while mercury, graphite and copper

are conductors.3. Induction, conduction, magnet, charged body.

Ans. Magnet. Induction, conduction and charged body are all related tostatic electricity while magnet is not.

4. Mercury, graphite, oxygen, human body, steel.Ans. Oxygen. Oxygen is an insulator while mercury, graphite, human body

and steel are conductors.5. Sulphur, rubber, wood, hydrogen, iron.

Ans. Iron. Iron is a conductor while sulphur, rubber, wood and hydrogenare insulators.

6. Electron, atom, proton, neutron.Ans. Atom. An atom is the smallest unit of an element, which may or may

not have an independent existence, but always take part in a chemicalreaction. Atom is made up of electron, proton and neutron.

7. Chloride ion, oxide ion, nitride ion, magnesium ion.Ans. Magnesiumion. Magnesium ion is positively charged ion while

chloride ion, oxide ion and nitride ions are negatively charged ions.

VI. Give reasons for the following.1. When you touch the metal disc of an electroscope with a charged

glass rod the metal leaves diverge.Ans. When we touch the metal disc of an electroscope with a charged glass

rod the metal leaves diverge because of the transfer of free electrons.2. One should not use an umbrella while crossing an open field in a

thunderstorm.Ans. One should not use an umbrella while crossing an open field in a

thunderstorm because when a charged cloud passes over anumbrella, it induces a charge in it, and there is an electrical dischargebetween an umbrella and the cloud. Which may be very dangerous.

3. In an electroscope the disc, the connecting rod and the leaves are allmade of metal.





Ans. In an electroscope the disc, the connecting rod and the leaves are allmade up of metals because metals are good conductors of electricity.

4. The metal wires in the bottom part of the electroscope are earthed.Ans. The metal wires in the bottom part of the electroscope are earthed

because if there is any charge inside the bottle, it immediately flowsto the earth making the inside of the bottle neutral.

5. The diverging leaves of a charged electroscope collapse, the momentyou touch the disc of the electroscope with your finger.

Ans. The diverging leaves of a charged electroscope collapse, the momentyou touch the disc of the electroscope with your finger because noelectric charge is present on it as the charge will flow from hand toearth through the body.

VII. State the difference between the following.1. Proton and electron.

Ans. Proton is a positively charged particle having one unit positive chargeand has a mass almost equal to mass of one atom of hydrogen.While electron is a negatively charged particle having one unit negativecharge and has a mass of 1/1837 times the mass of one atom ofhydrogen.

2. The charge carried by a glass rod rubbed with silk and charge carriedby ebonite rod rubbed with fur.

Ans. When a glass rod is rubbed with silk, a positive charge develops onthe glass rod and a negative charge is developed on silk. When anebonite rod is rubbed with fur, a negative charge develops on eboniterod and a positive charge develops on fur.

3. Like charges and unlike charges.Ans. Like charges repel each other while unlike charges attract each other.

4. Static electricity and current electricity.Ans. When an electric charge is at rest, it is known as static electricity

when an electric charge is in motion, it is referred to as a currentelectricity.

5. Coductors and insulators.Ans. The substances which have a large number of free electrons such





that they start drifting from one end of a substance to the other end,when it is connected to some source of electricity are calledconductors.While the substances which have a few free electrons, such thatthey do not easily drift from one end to the other end, when connectedto some source of electricity are called insulators.

VIII. Answer the following questions.1. Describe an experiment to show:

(a) Like charges repel each other.(b) Unlike charges attract each other.

Ans. (a) Experiment to show that likecharges repel each other:Take a glass rod and rub it withsilk and suspend it freely by a silkthread. Near this Suspended rod,bring another glass rod which isrubbed with silk. It is observed thatsuspended glass rod gets repelled.

(b) Experiment to show that unlikecharges attract each other:Take a glass rod and rub it withsilk and suspend it freely by a silkthread. Bring near it an ebonite rodwhich is rubbed with cat’s skin. Itis observed that glass rod isattracted by ebonite rod.

2. Briefly describe Rutherford’s structure of atom.Ans. Rutherford’s structure of atom:

(i) An atom consists of threesubatomic particles neutrons,protons and electrons.

(ii) Neutron has no charge, protonhas one unit postive charge andelectron has one unit negativecharge.





(iii) Protons and neutrons form the central core of atom which iscommonly called nucleus.

(iv) The electrons revolve around the nucleus in fixed orbits.(v) As the atom of a normal element is electrically neutral therefore,

it is believed that the number of protons in an atom is equalto the number of electrons.

3. Why does a glass rod get positively charged, when rubbed withsilk? Explain on the basis of electron transfer.

Ans. The glass rod transfers its electrons to the silk when it is rubbedwith silk. Thus, glass rod has less number of electrons than thenumber of protons in the nucleus. Thus, on the whole, the glass rodgets positively charged.

4. Why does an ebonite rod get negatively charged, when rubbed withfur? Explain on the basis of electron transfer.

Ans. Ebonite rod gains electrons from the fur when it is rubbed with fur.Thus ebonite has more negative charges as compared to positivecharges in the nucleus. Thus, on the whole, ebonite rod gets negativelycharged.

5. state the mass and charge on(a) electron (b) proton (c) neutron

Ans. Charge Mass(a) Electron 1 unit negative charge 1/1837 times that of mass

of one hydrogen atom.(b) Proton 1 unit positive charge Almost equal to hydrogen

atom.(c) Neutron No electric charge Almost equal to hydrogen

atom.6. What are conductors? Define on the basis of structure of atom.

Give four examples of different classes of conductors.Ans. A substance, which has a large number of free electrons, such that

they start drifting from one end of a substance to the other end,when it is connected to some source of electricity, is called conductor.Four examples of different classes of conductors are:





(i) Metals: Iron, copper, gold, silver, etc.(ii) Solution of acids in water: Dil hydrochloric acid, dil nitric

acid, dil sulphuric acid and carbonic acid, etc.(iii) Solution of alkalies in water: Sodium hydroxide, ammonium

hydroxide, calcium hydroxide, potassium hydroxide, etc.(iv) Solution of soluble salts in water: Sodium chloride,

potassium nitrate, etc.7. What are insulators? Define on the basis of structure of atom. Name

six insulators.Ans. A substance, which has a few free electrons, such that they do not

easily drift from one end of the substance to the other end, whenconnected to some source of electricity is called insulator.For example, alcohol, ether, sugar, starch, wool, fur etc.

8. How can you use gold leaf electroscope to detect the charge on abody?

Ans. Gold leaf electroscope (G.L.E) is used to detect the charge on abody.Take a glass rod and rub it with silk. Touch the rubbed end of theglass rod with the disc of G.L.E. It is seen that leaf of G.L.E. divergesoutward.When the glass rod touches the brassdisc of G.L.E, then free electrons fromit flow to the glass rod. This causes adeficiency of electrons on the brassdisc. To make up for this deficiencythe electrons from leaves flow towards the disc. This, in turn, causeselectron deficiency on the leaves. Asleaves are positively charged, theyrepel each other, and hence diverge.Thus, we can say that if a body, on touching disc of gold leafelectroscope makes its leaves diverge, then that body is charged.

9. How can you use gold leaf electroscope to detect the nature ofcharge on a body?





Ans. We take a G.L.E. and charge it positively or negatively by touchingits disc with a charged rod. Suppose we have a negatively chargedG.L.E. Now touch the given charged body to the disc of negativelycharged G.L.E.If the leaves of G.L.E. diverge further,then the body is negatively charged.It is because, similar charges repel eachother.If the leaves of G.L.E. collapse, thenthe body is positively charged. It isbecause some charges from negativelycharged G.L.E. will flow to thepositively charged body.

10. (a) State the charge present on the glass rod and silk when rubbedwith each other

(b) State the charge present on the ebonite rod and cat’s skin whenrubbed with each other.

Ans. (a) Glass rod gets positively charged and silk gains electrons sogets negatively charged after rubbing.

(b) Ebonite rod gets negatively charged and cat’s skin gets positivelycharged when rubbed with each other.

11. What is an electroscope? Draw a neat diagram of gold leafelectroscope.

Ans. An electroscope is a device used for detecting electric charges, andfinding the nature of electric charges.

Gold leaf electroscope





12. What causes thunder and lightning?Ans. When two clouds carrying opposite charges approach each other, a

charge flows from one to the other through the air. This electricaldischarge which can be seen as a dazzling flash of light is calledlightning.When lightning occurs the air suddenly gets very hot and expands.It subsequently cools and contracts. This sudden and tremendousexpansion and contraction of air produces an explosion of loudsound which is called thunder.

13. Explain how a lightning conductor works?Ans. A lightening conductor is made of copper rod, on the one end of

which are provided sharp copper points. It is installed on the highestpoint of the building. Its lower end is connected to a thick copperstrip which is attached to flat copper plate and buried deep insidethe earth.When lightning strikes the sharp points of lightning conducto, all theelectric discharge flows into the earth through the copper rod wire,without damaging the building.




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Ch-6 · Title: Ch-6.pmd Author: gbp Created Date: 8/20/2009 5:57:29 PM